Project abstract Despite modern immunosuppressive therapies, patients with systemic lupus erythematosus (SLE) remain at high risk for progressive organ damage, emphasizing the need for better, targeted treatments for this disease. In addition to the production of pathogenic autoantibodies, recent studies have demonstrated that B cells can promote lupus pathogenesis by initiating immune tolerance breaks and facilitating the generation of spontaneous germinal centers (GC). In this context, distinct costimulatory receptor families have been linked with the pathogenesis of autoimmunity. However, despite compelling preclinical data in SLE and clinical benefit in other autoimmune diseases, costimulatory blockade with CTLA4-Ig (Abatacept) failed to control disease in lupus clinical trials. These data emphasize that our understanding of the cell-intrinsic mechanisms whereby B7:CD28 costimulatory signals impact autoreactive B cell activation in lupus is incomplete. In this project, we will use well-characterized murine lupus models and the novel application of chimeric antigen receptor (CAR) T cell technology to dissect the immune mechanisms underlying the initiation, propagation and cellular output of extra-follicular (EF) vs. GC B cell activation pathways in SLE. In Aim 1, we will study whether pathogenic autoantibodies can be generated via an EF B cell activation pathway in a T cell-dependent, but CD28 independent, manner. In Aim 2, we will test whether B cell costimulatory signals promote the initiation or maintenance of autoimmune GC responses. Finally, in Aim 3, we will test whether another costimulatory receptor pair, ICOS:ICOS ligand, compensates for loss of CD28 signals during lupus pathogenesis. Together, these studies promise to advance our understanding of lupus pathogenesis and may inform the design of future human clinical trials of costimulatory blockade in SLE and other humoral autoimmune diseases.